[PATCH 4/4] can: mcba_usb: populate ndo_change_mtu() to prevent buffer overflow

[Vincent Mailhol <mailhol@kernel.org>]

Sending an PF_PACKET allows to bypass the CAN framework logic and to
directly reach the xmit() function of a CAN driver. The only check
which is performed by the PF_PACKET framework is to make sure that
skb->len fits the interface's MTU.

Unfortunately, because the mcba_usb driver does not populate its
net_device_ops->ndo_change_mtu(), it is possible for an attacker to
configure an invalid MTU by doing, for example:

  $ ip link set can0 mtu 9999

After doing so, the attacker could open a PF_PACKET socket using the
ETH_P_CANXL protocol:

	socket(PF_PACKET, SOCK_RAW, htons(ETH_P_CANXL))

to inject a malicious CAN XL frames. For example:

	struct canxl_frame frame = {
		.flags = 0xff,
		.len = 2048,
	};

The CAN drivers' xmit() function are calling can_dev_dropped_skb() to
check that the skb is valid, unfortunately under above conditions, the
malicious packet is able to go through can_dev_dropped_skb() checks:

  1. the skb->protocol is set to ETH_P_CANXL which is valid (the
     function does not check the actual device capabilities).

  2. the length is a valid CAN XL length.

And so, mcba_usb_start_xmit() receives a CAN XL frame which it is not
able to correctly handle and will thus misinterpret it as a CAN frame.

This can result in a buffer overflow. The driver will consume cf->len
as-is with no further checks on these lines:

	usb_msg.dlc = cf->len;

	memcpy(usb_msg.data, cf->data, usb_msg.dlc);

Here, cf->len corresponds to the flags field of the CAN XL frame. In
our previous example, we set canxl_frame->flags to 0xff. Because the
maximum expected length is 8, a buffer overflow of 247 bytes occurs!

Populate net_device_ops->ndo_change_mtu() to ensure that the
interface's MTU can not be set to anything bigger than CAN_MTU. By
fixing the root cause, this prevents the buffer overflow.

Fixes: 51f3baad7de9 ("can: mcba_usb: Add support for Microchip CAN BUS Analyzer")
Signed-off-by: Vincent Mailhol <mailhol@kernel.org>
---
 drivers/net/can/usb/mcba_usb.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/drivers/net/can/usb/mcba_usb.c b/drivers/net/can/usb/mcba_usb.c
index 41c0a1c399bf36104fbf8991b537acdb01e7e950..1f9b915094e64de875adff35026e9cd3beb0410d 100644
--- a/drivers/net/can/usb/mcba_usb.c
+++ b/drivers/net/can/usb/mcba_usb.c
@@ -761,6 +761,7 @@ static const struct net_device_ops mcba_netdev_ops = {
 	.ndo_open = mcba_usb_open,
 	.ndo_stop = mcba_usb_close,
 	.ndo_start_xmit = mcba_usb_start_xmit,
+	.ndo_change_mtu = can_change_mtu,
 };
 
 static const struct ethtool_ops mcba_ethtool_ops = {

-- 
2.49.1